Commit Message

This is a note to let you know that I have just added a patch titled
ptrace/x86: Partly fix
to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree
which can be found at:
http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue
If you, or anyone else, feels it should not be added to this tree, please
reply to this email.
For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable
Thanks.
-Luis
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From 04a8df512c72f3373a05b11eaf2bb6d7457545dd Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@redhat.com>
Date: Sat, 11 Aug 2012 18:06:42 +0200
Subject: [PATCH] ptrace/x86: Partly fix
set_task_blockstep()->update_debugctlmsr() logic
commit 95cf00fa5d5e2a200a2c044c84bde8389a237e02 upstream.
Afaics the usage of update_debugctlmsr() and TIF_BLOCKSTEP in
step.c was always very wrong.
1. update_debugctlmsr() was simply unneeded. The child sleeps
TASK_TRACED, __switch_to_xtra(next_p => child) should notice
TIF_BLOCKSTEP and set/clear DEBUGCTLMSR_BTF after resume if
needed.
2. It is wrong. The state of DEBUGCTLMSR_BTF bit in CPU register
should always match the state of current's TIF_BLOCKSTEP bit.
3. Even get_debugctlmsr() + update_debugctlmsr() itself does not
look right. Irq can change other bits in MSR_IA32_DEBUGCTLMSR
register or the caller can be preempted in between.
4. It is not safe to play with TIF_BLOCKSTEP if task != current.
DEBUGCTLMSR_BTF and TIF_BLOCKSTEP should always match each
other if the task is running. The tracee is stopped but it
can be SIGKILL'ed right before set/clear_tsk_thread_flag().
However, now that uprobes uses user_enable_single_step(current)
we can't simply remove update_debugctlmsr(). So this patch adds
the additional "task == current" check and disables irqs to avoid
the race with interrupts/preemption.
Unfortunately this patch doesn't solve the last problem, we need
another fix. Probably we should teach ptrace_stop() to set/clear
single/block stepping after resume.
And afaics there is yet another problem: perf can play with
MSR_IA32_DEBUGCTLMSR from nmi, this obviously means that even
__switch_to_xtra() has problems.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
arch/x86/kernel/step.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
--
1.8.1.2